Photographic process employing bichromated colloids



Patented Mar. 12, 1935 PATENT OFFICE PHOTOGRAPHIC PROCESS EMPLOYING BICHROMATED (COLLOIDS Frank William Sharp, London, England, assignor to The Autotype Company Limited, London,

England, a company of Great Britain No Drawing.

Application December 16, 1933,

Serial No. 702,787. In Great Britain March 4,

10 Claims.

This invention comprises improvements in photographic processes employing bichromated colloids and relates particularly to what is known as the bichromate process in which colloidal base such as gelatine is impregnated with a bichromate solution so as to become tanned (i. e. of reduced solubility in water) under the influence of light.

It has previously been proposed to increase the speed of the tanning action by using metal salts in the bichromating bath for gelatine films. Very little success was met with, however, in previous attempts, as it was found that such bichromated films prepared with metal salts in the bichromating bath although more rapid in tanning under the influence of. light compared with plain bichromated films were found to be more or less fogged when exposed and developed and such fogging rapidly increased if the papers were stored for a short time after sensitization.

It is an object of the present invention to provide a process for the production of a bichromate film which has increased speed compared with plain bichromated gelatine and does not fog.

The present invention accordingly comprises a process for the production of colloidal-base bichromate-sensitized films rendered more rapid by the inclusion of a soluble metal salt, (for example a salt of a rare earth) characterized by the fact that the soluble metal salt is incorporated with the colloidal base in its unsensitized state and the product dried prior to sensitization with bichromate.

The colloidal films according to this invention can withstand long'storage without. deterioration while impregnated with the soluble metallic salts before sensitization and, in addition, they can be stored for a time after sensi tization if desired without becoming insoluble or fogged as they would do were bichromation and impregnation with salts carried out simultaneously.

Preferably, the salt employed is a salt (for example a chloride) of a rare earth metal the chromate of which precipitates approximately at a pH value lying between 7 and 8.5 measured at infinite dilution. The salts of cerium (cerous) and lanthanum are found to be particularly suitable.

Alternatively, or in addition,the soluble salt employed may comprise a salt of one or more of the group of metals copper, cobalt or nickel.

Preferably the soluble metal salt is incorporated with the colloidal base before it is spread out, as a film.- i

It is thought that the action of light upon a potassium bichromate-sensitized gelatine film may be accounted for on the assumption that 5 the bichromate splits up into an insoluble salt which may be regarded as chromium chromate together with soluble potassium chromate. The chromium chromate eifects the tanning of the gelatine and inhibits the solution thereof. 10 The potassium chromate formed simultaneously with the chromium, chromate therefore has served no useful purpose. However, in the presence of the metal salts of one or more of the metals specified above, the potassium chromate reacts therewith to form an insoluble reaction product which has a tanning action. It appears that the reaction product formed comprises the chromate or a basic chromate of the metal salt employed.

, 20 In one example of the process according to the invention an emulsion is formed by dispersing approximately 3% by weight of dry water-insoluble pigment in a 25% solution of gelatine. The mixture is agitated with the addition of a quantity of cerous chloride dissolved in a small quantity of water sufiicient to yield a 2% solution, by weight of the whole emulsion. The emulsion is then evenly coated upon one side of paper of the kind usually sold for preparing resists on copper for etching for photogravure or for the process known as the carbon process, and the coating is allowed to dry.

Ina second form according to the invention an emulsion is formed by dispersing about 3%, by weight, of dry pigment into a 25% solution of gelatine. With this emulsion is added sufficient copper sulphate (or chloride) dissolved in a small quantity of water to give 1% solution, by weight, on the bulk of emulsion, which is afterwards coated on to one side of a photo paper. Alternatively, in the case of nickel and cobalt, about 2% of the sulphate or chloride is added. 45

It is found that when the paper prepared by either of the examples described above is subsequently sensitized by soaking in bicromate solution, for example pure potassium, sodium or am monium bichromate, the rapidity of the tanning action shows a marked increase compared with that obtained using paper not containing salts according to the present invention. When a salt of one or more of the rare earth metals is used the rapidity of the tanning is increased approximately three times, whilst with copper, cobalt or nickel salts the rapidity of the tanning action is approximately twice that of the usual paper, and thus the length of exposure to light for a given degree of tanning can be correspondingly reduced. a

It isimportant thatcoated paper containing salts according to the present invention should be sensitized in pure bichromate and not bichromate to which ammonia has been added, since a reaction product would be formed which would tan the gelatine in the presence of the above mentioned salts even Without exposure to light.

The light-sensitive paper may then be exposed behind a negative or behind a screen and positive according to the particular printing process in which the paper is being used.

I claim 1. A method of producing colloidal-base films which can berendered light-sensitive by treat,- ment with a solution of a bichromate, consisting in introducing into a colloidal base a soluble metal salt selected from the group consisting of copper, cobalt, nickel, and the rare earths and drying the colloidal base While in an unsensitized state. I a

2; A method of producing colloidal-base films which can be rendered light-sensitive by treatment with a solution of a bichromate,- consisting in introducing a soluble salt'cf a r'areearth into the colloidal-base and drying thelatter in an unsensitized state. I

3L A method of producing colloidal-base films asclaimed in; claim 2, wherein the soluble salt is a salt of a rare earth metal, the chromate of which precipitates approximately at a pH value lying between 7 and 8.5 measured at infinite I dilution.

4. A method of producing colloidal-base films as claimed in claim 2 whereinthe solublesalt is p r v a soluble chloride -of a rare earth metal, the

chromate of which precipitates approximately ata pH value lying between 7 and 8.5 measured at infinite dilution.

5. A method of producing colloidal-base films as claimed in claim 2, wherein the soluble salt is cerous chloride. 1

6. A method. of producing colloidal-base films as claimed in claim 2, wherein soluble salt is a salt of lanthanum.

7. A method of producing colloidal-base films which can be rendered light-sensitive by treatment with a solution of a bichromate, consisting in introducing a soluble salt of one of the metals comprising the group copper, nickeljand cobalt latterin intothe colloidal base and drying the an unsensitized state.

8. A method of producing colloidal-basefilms as claimed in claim2, wherein the soluble salt of alrare earth is introduced into the colloidal base before it is spread out as a'film.

9. A method of producing colloidal base films as claimed in claim '7, wherein the soluble metal salt is a sulphate. r 1 v 9 10. A method of producing colloidal base films as claimed in claim '7, wherein the soluble metal salt is a chloride. u I T FRANK WILLIAMVSVHARP. 

